<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    /* 上下margin折叠 取大值 */
    .box1 {
      height: 100px;
      background-color: #f00;
      margin-bottom: 30px;
    }

    .box2 {
      height: 100px;
      background-color: #0f0;
      margin-top: 50px;
    }
  </style>
</head>

<body>
  <div class="box1"></div>
  <div class="box2"></div>
</body>

</html>